Dynamoelectric machines such as motors and generators and other rotating machines such as gears and bearing systems are widely employed in industrial and commercial facilities. These machines are relied upon to operate with minimal attention and provide for long, reliable operation. Many facilities operate several hundreds or even thousands of such machines concurrently, many of which are integrated into a large interdependent process or system. Like most machinery, at least a small percentage of such equipment is prone to failure. Some of such failures can be attributed to loss of lubrication, incorrect lubrication, lubrication breakdown or lubrication contamination.
Depending on the application, the failure of a machine in service can possibly lead to system or process down time, inconvenience, material scrap, hazardous material cleanup and possibly even a dangerous situation. Thus, it is desirable to diagnose the machinery for possible failure or faults early in order to take preventive action and avoid such problems. Absent special monitoring for certain lubrication problems, the problem may have an insidious effect in that although only a minor problem on the onset the problem could become serious if not detected. For example, bearing problems due to inadequate lubrication, lubrication contamination or other causes may not become apparent until irreversible damage has occurred.
Proper lubrication facilitates the extension of machinery life. For example when motor lubrication is continuously exposed to high temperatures, high speeds, stress or loads, and an oxidizing environment, the lubrication will deteriorate and lose its lubricating effectiveness. The loss of lubricating effectiveness will affect two main functions of a lubrication system, namely: (1) to reduce friction; and (2) to remove heat. Continued operation of such a degraded system will result in even greater heat generation and accelerated system degradation. To protect the motor, the lubrication should be changed in a timely fashion. However, a balance must be struck--on one hand it is undesirable to replace an adequate lubricant, but on the other hand it is desired to replace a lubricant that is in its initial stages of breakdown or contamination before equipment damage occurs. Since each particular application of a lubricant is relatively unique with respect to when the lubricant will breakdown or possibly become contaminated, it becomes necessary to monitor the lubricant.
Various techniques for analyzing lubricants are known. For example, measuring a dielectric constant change in the lubricant or recording a thermal history of the lubricant have been employed for monitoring the lubricant's condition. However, these methods require the use of the same lubricant or assume no machinery malfunctions throughout the measurements. Furthermore, these monitoring techniques typically require that a sample of the lubrication be extracted and analyzed using laboratory grade equipment to determine the condition of the lubricant.
Using two-electrode type sensors to measure conductivity changes has been tried. The selectivity of such sensors is generally not sufficient to differentiate between a new lubricant and deteriorated used lubricant. This is because current collected from measuring both new and used lubricants is high. Consequently, it is often difficult for such sensors to distinguish between new and degraded lubricants because all electroactive species are collected with near equal efficiency. Furthermore, it is known that the dielectric constant of different brands of lubricants differ from each other. Therefore, it is difficult to find a dielectric constant value at which all brands of lubricants are definitely bad.
In view of the above, there is a need for an improved sensor for detecting a health condition of a lubricant.